MANUAL OF NATDRB STUDY. 29 



Now what happens to the air in the lungs when 

 they are pressed together by the ribs? What hap- 

 pens to the blood pump? What effect would that 

 have upon the supply of blood to the system? 

 Why do we need the blood? Why do we need air? 



Teachers must explain that the blood must be 

 purified and that our breathing helps to do that 

 thing. That there are blood tubes and air pas- 

 sages, capillaries and millions of air cells in the 

 lungs, and that the blood trades off its impurities 

 for the oxygen of the air. The lungs may be com- 

 pared to a market house in that it is a place for 

 trading. Which makes the best trade, the blood 

 or the air? 



When does a gardner find it hard to get rid of 

 his cabbage or radishes? When the market house 

 is already full of such vegetables. When does 

 the blood find it difl&cult to get rid of its impurities? 

 When the air in the lungs is already full of im- 

 purities. 



Why is plenty of pure air then necessary? How 

 does stooping over while sitting at the desk affect 

 the amount of pure air in the lungs? How does 

 smoking cigarettes or tobacco of any kind affect the 

 purity of air we breathe? How do close, ill-ven- 

 tilated school rooms affect the purity of air ? Then 

 how would that affect the purity of the blood ? 



